Tailored structural and magnetic traits of hard/soft ferrite nanocomposites: Effect of cerium substitution

Hard/soft (H/S) CoCe _x Fe_{2- x} O₄/Ni_0.3Cu_0.3Zn_0.4Fe₂O₄ (x ≤ 0.025) spinel nanocomposites (H/S Ce→CFO/NCZFO (x ≤ 0.025) SNCs) were obtained via ultrasonic irradiation route. X-ray diffraction (XRD) patterns confirmed the formation of a single-phase cubic spinel structure for all compositions, with the average crystallite size ( D _p ) ranging between 10.8 and 19.8 nm. Electron microscopy images revealed clusters of spherical nanoparticles, while energy-dispersive X-ray (EDX) mapping verified the homogeneous distribution of all constituent elements. Vibrating sample magnetometry revealed that the nanocomposites exhibit superparamagnetism at ambient temperature. This was characterized by smooth M-H curves and single-peak in their demagnetization differential curves, which indicate an effective exchange coupling between the two ferrite components. At 20 K, the combinations revealed ferrimagnetic behaviors. While the pristine H/S ( x = 0.00) displayed a smooth M-H curve and maintained a single-phase-like switching behavior, the Ce-doped H/S ferrite-based samples ( x ≥ 0.010) exhibited kinks in their M-H curves and multiple peaks in their demagnetization differential curves, indicating a weakening of the exchange coupling at low temperatures. Substitution with Ce ions led to a systematic decrease in saturation magnetization ( M _s ) and magnetic moment ( n _B ), while the coercivity ( H _c ) increased significantly at low temperatures, reflecting enhanced magnetocrystalline anisotropy. The measurements of magnetization versus temperature confirmed the transition from a superparamagnetic state to a ferrimagnetic state, with blocking temperatures ( T _B ) observed to increase from 149.1 K ( x = 0.015) to 162.2 K ( x = 0.025). These results highlight the role of Cerium in tailoring the magnetic hardness and thermal stability of hard/soft ferrite systems.